Cover*

The nucleolus, a prominent organelle inside the nucleus, has fascinated cell biologists for well over a century. Carnoy (1884) gave credit to Fontana (1781) as having first visualized the nucleolus. However, whether Fontana really saw a nucleolus has been questioned in a scholarly historical review by Franke (1988). Wagner (1835) discovered the nucleolus ("keimfleck" or "macula germinativa") in oocyte germinal vesicles, and the following year it was also described by Valentin (1836a,b), a student of Purkinje, who subsequently named it "the nucleolus" or "kernkörperchen" (Valentin, 1839). Schleiden (1838), Schwann (1839), and many others to follow observed the nucleolus, Hertwig (1876) commented that it was the most important element of the nucleus, and Flemming (1882) concluded that it was in all cells. A voluminous literature on the subject already existed 100 years ago when Montgomery (1898) wrote his classic treatise on the nucleolus. The cover figure shown here is taken from that work, showing nuclei with nucleoli (n) of various shapes, sizes, and morphology from Gregarines (Figures 26, 28, and 31), brain cells (Figures 46, 49, and 94-96), and oocytes (Figures 130, 223, and 241). The number of nucleoli was variable, from one to a few in diploid cells to hundreds in oocytes. Nucleolar function was not yet established, but observations of a nucleolar vacuole (n. Vac) suggested to Montgomery and others that it might be an excretory organ. Different views on function were presented by Wilson (1896), who wrote, "we can hardly doubt the conclusion of Häcker that the nucleoli of germ cells are by-products of the nuclear action, derived from the chromatin." How right he was, as we now know that the nucleolus contains ribosomal DNA (first seen by Painter in 1942) and is the site of ribosomal RNA transcription, processing, and biogenesis of the ribosome (reviewed in Gerbi et al, 1990). The next hundred years are likely to see new functions emerge for the nucleolus, as it has been implicated in modification of certain spliceosomal small nuclear RNAs, processing and perhaps modification of transfer RNA, and export of mRNA and RNA of the signal recognition particle (reviewed in Pederson, 1998). Telomerase RNA has been found in nucleoli, and the nucleolus may even be linked with mechanisms of aging (reviewed in Pederson, 1998). The cover figure is from Montgomery (1898) (© 1898, Wiley-Liss, Inc.) and is reprinted with permission of Wiley-Liss, Inc. a division of John Wiley & Sons, Inc.

Susan A. Gerbi and T. Sascha Lage

*This cover celebrates the 100th anniversary of T.H. Montgomery's classic treatise on the nucleolus and the 70th birthday of Joe Gall, whose multifaceted contributions have enhanced our understanding of this subject; an article by the authors on the nucleolus appears in this issue.